KT77 SE Stereo Amplifier

     KT77 single ended stereo project. I have to say, it sounds far better than I expected for a low power amp. I would have never believed 4 watts of tube power could sound so nice without hearing it myself. No flaws in this single ended amp. I am impressed with the amount of clean bass and clarity of the high frequencies. Being that I have never heard a single ended stereo amplifier ( well that I can remember in the last 50 years) it had been on my mind for awhile. I ended up using couple of new release Gold Lion KT77’s, Audiocap Theta coupling capacitors, Transcendar output transformers, and a vintage Olson electronics power transformer.

Chart KT77

     When figuring out some of the final values for a SE amplifier it helps to use simulation software to get in the ballpark. Using Glassware Audio’s Vintage SE Amp Cad software I was able to see the results of using KT77’s in this amp. It is a handy piece of software. You can enter in values including transformer specs to see what the scenario results would be. Trancendar’s web site listed detailed specs on the transformer which helped a lot.

KT77 stereo schematic

     KT77 amplifier schematic.  The coupling capacitors in this project are Audiocap Thets’s. I usually am happy using Mallory 150P’s, but I thought I’d try the Audiocaps. As I usually do,  the pre-amp tubes have regulated DC on the filaments.  The amp is dead quiet, nice. With only a 6.3 volt tap on the power transformer I would usually have used a voltage doubler circuit with a regulator and the pre-amp tubes in series. But, the power transformer I used was a new old stock made for Olson Electronics, remember them?  The secondary voltages were a little higher I’m guessing because it was made for 110 – 115 volts. So there was enough headroom to uses a full wave bridge and a regulator to push out a solid 6.2 volts DC for the pre-amp tubes.  The output tube filaments are running at 6.6 volts which is not as high as the 6.9v limit on the power tubes.

KT77 amplifier

     I find myself listening to Triode mode most of the time, though Ultra-linear and Pentode mode sound nice to. The tubes you see here are EL34’s which I used before the KT77’s arrived. 

KT77 chassis
KT77 chassis 2

     The chassis is a standard Bud 12×12 aluminum chassis with a dark metallic paint.  Drilling and punching the chassis is the least fun part for me. The output transformers are Transcendar  TT-336 OT.

KT77 inside chassis
KT77 se2 amp rear

Inside view of the chassis and rear view of the chassis with the usual hardware.

6L6 Push Pull amplifier

6L6 Stereo Front

      I wanted to build a good sounding 6L6 amplifier in the 20 watt range with minimal components. I believe less is more to attain good sound. With only 3 coupling capacitors involved per channel it delivers a solid 17 watts per channel at 1.5V input. Using Sovtek 6L6WXT output tubes, NOS Tung-Sol JAN 6SL7WGT preamp tubes, Edcor transformers, and regulated DC on the 6SL7 filaments.

6l6 Stereo Amplifier

     Here is the amplifier schematic.  It is based on an old AcroSound circuit. The PC boards are the same boards that I used in my 6V6 amp and were modified with a tag board posts to accommodate the AcroSound circuit. The original AcroSound circuit was a tetrode configuration which I changed to an ultraliner configuration. The original description describes a circuit that with a quality output transformer and a stable feedback arrangement permits a clean output with excellent distortion and transient characteristics. The original designed load line required a AcroSound TO-280 9K output transformer. I substituted an Edcor 8.6K which worked nicely.

6L6 chassis 1

     The chassis is a standard Hammond aluminum chassis.  After gathering your components you can layout the chassis by leaving the protective peel off plastic on. I use a fine tip permanent marker to layout the holes.  You can mark your holes for drilling with a spring loaded center punch.  I make the larger holes with a vintage radio punch set which does a nice job on the transformer wiring holes and tube socket holes.

6L6 inside chassis

 Inside view of the chassis. The power supply sits in the middle.

6L6 chassis 3
6L6 chassis side
6L6 Chassis Rear

EL34 vacuum tube amplifier

      El34 Push Pull Stereo Amplifier. I used Edcor CXPP-25-8-5K transformers, Hammond 159T choke, TAD EL34B STR Premium tubes, two 12AU7’s and one 12AX7 while implementing fixed bias, a Rubli high voltage turn on delay board, and regulated DC for the pre-amp tube filaments.

El34 Stereo Amplifier
El34 Schematic

     Here is the amplifier schematic.  For each channel, half of a 12AX7 provides signal for a 12AU7 long tailed pair which provides drive for the EL34’s.  You can use your favorite schematic, customize an existing circuit as I often do, or design your own.

El34 Power Supply

     Power Supply.  Sometimes I use off the shelf parts and PC boards when building.  The power supply is a Glassware Audio PS-3 board with a few component changes. A Rubli high voltage delay board turns on the HV after the filaments warm up.  I had got the Rubli board to experiment with, This amp can have over 525v available until the tubes start conducting. It just makes sense that it will put less stress on the tubes on warm up. In a fixed bias amplifier with a high voltage delay, you can be sure that proper bias voltage is up before HV is applied to the tubes.

El34 chassis

      The chassis is a standard Hammond aluminum chassis.  After gathering your components you can layout the chassis by leaving the protective peel off plastic on. I use a fine tip permanent marker to layout the holes.  You can mark your holes for drilling with a spring loaded center punch.  I make the larger holes with a vintage radio punch set which does a nice job on the transformer wiring holes and tube socket holes.

El34 board top
El34 board bottom

     Sometimes I use PC boards. But you can use point to point wiring just as easy if you choose, or have a PC board made from your design. You also can purchase one of the pre-made boards as those such Pete Millett or others.

El34 inside chassis

      Bottom of the chassis.  You can see the Mallory 150p coupling caps, I like them, how they sound. PS-3 board is to the right. The Rubli board is mounted to the right out of sight to the right. Bias test points are located on the chassis at the rear of the PC board.

El34 chassis top 1

     Top of the chassis before the PC board was installed. You can see the holes for the bias adjustments near each EL34 socket, and the bias test points between the output transformers. Power transformer, power supply choke, and a separate filament transformer for the regulated DC pre-amp filament supply.

Ivan the cat inspector

And of course Ivan the cat inspecting my work!

Ivan napping after inspection

Satisfied with my work, Ivan takes a long nap!

6V6 Stereo Amplifier (Dynaco)

     This one is a 6V6 push pull stereo amplifier that is based on age old Dynaco 6V6 circuit that many have built. Edcor transformers were used and a Glassware Audio PS-14 supply that delivers high voltage and regulated DC to all tube filaments. The controls are a Glassware A3 Mini-stepped attenuator.

6v6 amplifier schematic

     This is 6V6 amplifier schematic. I used the standard 40% screen grid tap, but the 6V6 is supposed to be designed for 23% screen grid taps in ultralinear mode. See my Audionote Kit 4 clone where I used output transformers with 23% ultraliner taps.

6v6 chassis

     The chassis is a standard Hammond aluminum chassis.  After gathering your components you can layout the chassis by leaving the protective peel off plastic on. I use a fine tip permanent marker to layout the holes.  You can mark your holes for drilling with a spring loaded center punch.  I make the larger holes with a vintage radio punch set which does a nice job on the transformer wiring holes and tube socket holes. And yes on this one the boards are not removable because I used chassis mounted sockets then soldered the boards to the sockets. I doubt that they would ever need to be removed as all the components are accessible on the boards. Just something I tried.

6v6 chassis inside

     This is an inside view of the chassis. To the right is the power supply. hard to believe that I built this back in 2014. Time flies!

6v6 chassis top
6v6 chassis top

Sherwood S-5000 II Stereo


S-5000 II stereo amplifier

     This is a great Sherwood S-5000 II amplifier which uses 7868’s for the outputs. Beware if you find an amplifier that uses these tubes, you need to replace the sockets unless you find vintage tubes. The new 7868 tubes have much thicker pins and you will damage your expensive tubes trying to insert them into those older sockets designed for thinner pins.


S-5000 II stereo amplifier top

S-5000 II stereo amplifier tubes

S-5000 II stereo amplifier top 2

Pioneer SX-82 Vacuum Tube Stereo Receiver

Pioneer SX-82

      Some years ago, I restored a Pioneer SX-82 which is a late 60’s vintage AM/FM stereo. With it’s complement of 29 tubes, 28 watts per channel output, it was a nice challenge.  I replaced all the signal caps with Illinois caps (yellow), all the electrolytics, burnt wire-wound cathode resistors in the outputs, many of the tubes along with the Russian 6P15P-EV’s in the outputs. I uses those because the original 7189A’s are not available and current EL84’s would not be able to handle the power out.  It ended up with a nice sound and was worth the effort.

Pioneer SX-82 Top

      Because the original electrolytics were not available at the time the choices were to re-stuff them or replace and add what was needed,  I replaced them with some JJ’s and added the two beside the transformer as you can see from the photo.

SX-82 bottom1

     It takes patience, a steady had and time to replace all the capacitors in a receiver like this. The yellow caps are new.

SX-82 bottom 2
SX-82 bottom 3
SX-82 back
Pioneer SX-82 Specifications
Pioneer SX-82 schematic